It's not quite a case of Farmer Gavin picking up a hoe, but San Francisco has gone into the farming business on a swatch of city-owned watershed land in southern Alameda County.

Six acres of squash, tomatoes, peppers and melons are being grown for sale by four East Bay groups, according to a joint announcement yesterday by the San Francisco Public Utilities Commission and the nonprofit group, Sustainable Agriculture Education (SAGE).

The farm is part of a new concept called an AgPark being developed on 18 flat, fertile acres in the Sunol Valley east of Fremont and south of Pleasanton. Officially, it's called the Sunol Water Temple Agricultural Park.

The idea is to keep small farms close to the city while teaching urban dwellers, especially children, about how their food is grown, and providing outdoor recreation where they'll feel connected to the land. The farm is the first step toward a public park.

Educational programs connecting the farm to school districts, including San Francisco's, are in the works, according to SAGE President Sibella Kraus, founding director of the Francisco's Ferry Plaza Farmers Market and CUESA, the Center for Urban Education about Sustainable Agriculture.

"We see this as a model for what could happen on other public lands," Kraus said.

The Sunol Valley land, farmed long ago but more recently home to feral pigs and deer, is a small part of 40,000 acres in watershed land owned by the San Francisco PUC in Alameda County, she added.

After fencing the 18 acres and installing irrigation, four groups moved in to start farming in June.

Among them are staff and youth group members from the People's Grocery, the West Oakland community group, which is raising crops on 2 acres for its food distribution network.